refactor: standardize resource response format

Introduce ReadResourceContents type to properly handle MIME types in resource responses. Breaking change in FastMCP read_resource() return type.

Github-Issue:#152

src/mcp/server/fastmcp/server.py

@@ -20,6 +20,7 @@ from mcp.server.fastmcp.tools import ToolManager
 from mcp.server.fastmcp.utilities.logging import configure_logging, get_logger
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.server.lowlevel import Server as MCPServer
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
 from mcp.shared.context import RequestContext
@@ -197,7 +198,7 @@ class FastMCP:
             for template in templates
         ]
 
-    async def read_resource(self, uri: AnyUrl | str) -> tuple[str | bytes, str]:
+    async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
         """Read a resource by URI."""
 
         resource = await self._resource_manager.get_resource(uri)
@@ -206,7 +207,7 @@ class FastMCP:
 
         try:
             content = await resource.read()
-            return (content, resource.mime_type)
+            return ReadResourceContents(content=content, mime_type=resource.mime_type)
         except Exception as e:
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
@@ -608,7 +609,7 @@ class Context(BaseModel):
             progress_token=progress_token, progress=progress, total=total
         )
 
-    async def read_resource(self, uri: str | AnyUrl) -> tuple[str | bytes, str]:
+    async def read_resource(self, uri: str | AnyUrl) -> ReadResourceContents:
         """Read a resource by URI.
 
         Args:

src/mcp/server/lowlevel/helper_types.py

@@ -0,0 +1,9 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class ReadResourceContents:
+    """Contents returned from a read_resource call."""
+
+    content: str | bytes
+    mime_type: str | None = None

src/mcp/server/lowlevel/server.py

@@ -74,6 +74,7 @@ from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStre
 from pydantic import AnyUrl
 
 import mcp.types as types
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.models import InitializationOptions
 from mcp.server.session import ServerSession
 from mcp.server.stdio import stdio_server as stdio_server
@@ -253,20 +254,20 @@ class Server:
 
     def read_resource(self):
         def decorator(
-            func: Callable[[AnyUrl], Awaitable[str | bytes | tuple[str | bytes, str]]],
+            func: Callable[[AnyUrl], Awaitable[str | bytes | ReadResourceContents]],
         ):
             logger.debug("Registering handler for ReadResourceRequest")
 
             async def handler(req: types.ReadResourceRequest):
                 result = await func(req.params.uri)
 
-                def create_content(data: str | bytes, mime_type: str):
+                def create_content(data: str | bytes, mime_type: str | None):
                     match data:
                         case str() as data:
                             return types.TextResourceContents(
                                 uri=req.params.uri,
                                 text=data,
-                                mimeType=mime_type,
+                                mimeType=mime_type or "text/plain",
                             )
                         case bytes() as data:
                             import base64
@@ -274,34 +275,31 @@ class Server:
                             return types.BlobResourceContents(
                                 uri=req.params.uri,
                                 blob=base64.urlsafe_b64encode(data).decode(),
-                                mimeType=mime_type,
+                                mimeType=mime_type or "application/octet-stream",
                             )
 
                 match result:
                     case str() | bytes() as data:
-                        default_mime = (
+                        warnings.warn(
-                            "text/plain"
+                            "Returning str or bytes from read_resource is deprecated. "
-                            if isinstance(data, str)
+                            "Use ReadResourceContents instead.",
-                            else "application/octet-stream"
+                            DeprecationWarning,
-                        )
+                            stacklevel=2,
-                        content = create_content(data, default_mime)
-                        return types.ServerResult(
-                            types.ReadResourceResult(
-                                contents=[content],
-                            )
-                        )
-                    case (data, mime_type):
-                        content = create_content(data, mime_type)
-                        return types.ServerResult(
-                            types.ReadResourceResult(
-                                contents=[content],
-                            )
                         )
+                        content = create_content(data, None)
+                    case ReadResourceContents() as contents:
+                        content = create_content(contents.content, contents.mime_type)
                     case _:
                         raise ValueError(
                             f"Unexpected return type from read_resource: {type(result)}"
                         )
 
+                return types.ServerResult(
+                    types.ReadResourceResult(
+                        contents=[content],
+                    )
+                )
+
             self.request_handlers[types.ReadResourceRequest] = handler
             return func
 

tests/issues/test_152_resource_mime_type.py

@@ -6,6 +6,7 @@ from pydantic import AnyUrl
 from mcp import types
 from mcp.server.fastmcp import FastMCP
 from mcp.server.lowlevel import Server
+from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.shared.memory import (
     create_connected_server_and_client_session as client_session,
 )
@@ -98,9 +99,11 @@ async def test_lowlevel_resource_mime_type():
     @server.read_resource()
     async def handle_read_resource(uri: AnyUrl):
         if str(uri) == "test://image":
-            return (base64_string, "image/png")
+            return ReadResourceContents(content=base64_string, mime_type="image/png")
         elif str(uri) == "test://image_bytes":
-            return (bytes(image_bytes), "image/png")
+            return ReadResourceContents(
+                content=bytes(image_bytes), mime_type="image/png"
+            )
         raise Exception(f"Resource not found: {uri}")
 
     # Test that resources are listed with correct mime type

tests/server/fastmcp/servers/test_file_server.py

@@ -88,10 +88,10 @@ async def test_list_resources(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_dir(mcp: FastMCP):
-    files, mime_type = await mcp.read_resource("dir://test_dir")
+    res = await mcp.read_resource("dir://test_dir")
-    assert mime_type == "text/plain"
+    assert res.mime_type == "text/plain"
 
-    files = json.loads(files)
+    files = json.loads(res.content)
 
     assert sorted([Path(f).name for f in files]) == [
         "config.json",
@@ -102,8 +102,8 @@ async def test_read_resource_dir(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_file(mcp: FastMCP):
-    result, _ = await mcp.read_resource("file://test_dir/example.py")
+    res = await mcp.read_resource("file://test_dir/example.py")
-    assert result == "print('hello world')"
+    assert res.content == "print('hello world')"
 
 
 @pytest.mark.anyio
@@ -119,5 +119,5 @@ async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
     await mcp.call_tool(
         "delete_file", arguments=dict(path=str(test_dir / "example.py"))
     )
-    result, _ = await mcp.read_resource("file://test_dir/example.py")
+    res = await mcp.read_resource("file://test_dir/example.py")
-    assert result == "File not found"
+    assert res.content == "File not found"

tests/server/fastmcp/test_server.py

@@ -581,8 +581,8 @@ class TestContextInjection:
 
         @mcp.tool()
         async def tool_with_resource(ctx: Context) -> str:
-            data, mime_type = await ctx.read_resource("test://data")
+            r = await ctx.read_resource("test://data")
-            return f"Read resource: {data} with mime type {mime_type}"
+            return f"Read resource: {r.content} with mime type {r.mime_type}"
 
         async with client_session(mcp._mcp_server) as client:
             result = await client.call_tool("tool_with_resource", {})

tests/server/test_read_resource.py

@@ -0,0 +1,109 @@
+from pathlib import Path
+from tempfile import NamedTemporaryFile
+
+import pytest
+from pydantic import AnyUrl, FileUrl
+
+import mcp.types as types
+from mcp.server.lowlevel.server import ReadResourceContents, Server
+
+
+@pytest.fixture
+def temp_file():
+    """Create a temporary file for testing."""
+    with NamedTemporaryFile(mode="w", delete=False) as f:
+        f.write("test content")
+        path = Path(f.name).resolve()
+    yield path
+    try:
+        path.unlink()
+    except FileNotFoundError:
+        pass
+
+
+@pytest.mark.anyio
+async def test_read_resource_text(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(content="Hello World", mime_type="text/plain")
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.TextResourceContents)
+    assert content.text == "Hello World"
+    assert content.mimeType == "text/plain"
+
+
+@pytest.mark.anyio
+async def test_read_resource_binary(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(
+            content=b"Hello World", mime_type="application/octet-stream"
+        )
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.BlobResourceContents)
+    assert content.mimeType == "application/octet-stream"
+
+
+@pytest.mark.anyio
+async def test_read_resource_default_mime(temp_file: Path):
+    server = Server("test")
+
+    @server.read_resource()
+    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
+        return ReadResourceContents(
+            content="Hello World",
+            # No mime_type specified, should default to text/plain
+        )
+
+    # Get the handler directly from the server
+    handler = server.request_handlers[types.ReadResourceRequest]
+
+    # Create a request
+    request = types.ReadResourceRequest(
+        method="resources/read",
+        params=types.ReadResourceRequestParams(uri=FileUrl(temp_file.as_uri())),
+    )
+
+    # Call the handler
+    result = await handler(request)
+    assert isinstance(result.root, types.ReadResourceResult)
+    assert len(result.root.contents) == 1
+
+    content = result.root.contents[0]
+    assert isinstance(content, types.TextResourceContents)
+    assert content.text == "Hello World"
+    assert content.mimeType == "text/plain"